JPH0772613B2 - In-furnace denitration control method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a combustion control system for a boiler, and more particularly to a furnace denitration control method suitable for reducing nitrogen oxides generated during combustion.

[Conventional technology]

The conventional in-furnace denitration control system for this tumor is to reduce the nitrogen oxide (NOx) generation concentration of the boiler as much as possible, as described in JP-A-58-19608. It is what you are aiming for. The conventional control method described above is capable of effectively suppressing a transient increase in NOx when the load on the boiler changes.

[Problems to be Solved by the Invention]

However, in the above conventional control method, since the opening degree of the primary gas damper and the mixed gas damper is determined by each load zone by the boiler combustion test, the NOx value can be variably controlled in normal load operation. I couldn't do it.

Therefore, during the summer months, when an emergency NOx reduction command is issued due to the photochemical warning, the excess air amount (O
There was a problem that we had to deal with it by manually operating ( ₂ settings). Further, in the conventional NOx reduction device installed in the boiler flue, there is a problem that it is not possible to perform detailed NOx value control.

The object of the present invention is to solve the above-mentioned problems and to provide a NO
An object of the present invention is to provide a method for controlling NOx removal in a furnace that reduces NOx and enables fine control of the NOx value without providing an x reduction device.

[Means for Solving the Problems]

In order to achieve the above object, the present invention is a method for reducing nitrogen oxides by incorporating combustion air and exhaust gas into a combustion chamber and changing the composition of these to change the combustion state of the burner section. In the furnace denitration control method for variably controlling each flow rate of the mixed gas taken out from the combustion air and exhaust gas supplied to each stage burner, and the primary gas,
For each stage burner, the burner control and the air and gas flow rate control are stored in an integrated controller, and the air ratio to the load and the air ratio to the after air port are set in the controller. By setting several target indicators of nitrogen oxides and interpolating between them, the flow rate values of combustion air, mixed gas, and primary gas for the load and nitrogen oxide target indicators are set, and the set flow rates are set. It is a method characterized in that each of the flow rates is controlled so as to have a value.

[Action]

According to the above configuration, the air / gas flow rate control and the burner control are stored in the controller integrated for each stage burner, so that the signal exchange between air and gas (mixed gas, primary gas) is simplified. The timing of burner digestion and air / gas flow rate control can be finely controlled for NOx generation during burner digestion.

Then, the flow rates of the combustion air, the mixed gas, and the primary gas supplied to each stage burner are variably controlled by, for example, the burner inlet air damper and the after air port damper, while the flow rates are controlled by the NOx target index (target). However, it is not the NOx value itself, but the NOx value is regulated to a level of 0 to 100%, so it is called a target index.) And the NOx target is calculated by the air-gas ratio correction signal obtained from the boiler input command. Control to become an index.

That is, as shown in FIG. 6 and FIG.
Calculate the air ratio to the NOx master and the AA ratio (after air ratio, air ratio to the after air port) as follows. The air ratio and the AA ratio with respect to the load are given by the curve in Fig. 7, and the NOx target index of NOx master is 0%, 50%, 10%.
Set the curve for 0% with Fx60, 61, 62 in Fig. 6. Based on the NOx master 50%, the adder 67 subtracts 0 to 50%, and adds 50 to 100% to the air ratio, A
Find the A ratio. That is, at a certain load on the curve of FIG. 7, the difference between the NOx master 0% and 50% is obtained by the subtractor 65, and when the NOx master is between 0 and 50%, the difference of 0 to 50%. The (output of the subtractor 65) is obtained by the interpolation method using the multiplier 63, and the adder 67 subtracts it from the 50% value. At the time of 50 to 100%, the subtractor 66 and the multiplier 64 are used, and the adder 67 adds.

The dampers are operated to control the flow rates of the combustion air, the mixed gas, and the primary gas so that the flow rate value set in this way is achieved, and as a result, the combustion system control is performed even when the NOx value is changed. It can be operated as it is and it can be operated in a stable combustion condition.

〔Example〕

 Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 is a configuration diagram showing a schematic configuration of a thermal power plant to which the present invention is applied.

In FIG. 2, 1 is a boiler, 2 is a turbine, 3 is a generator, 4 is a water supply pump, 5 is a spray valve, 6 is a fuel valve, 7 is a forced draft fan, and 8 is a gas recirculation fan. If these are classified into systems, combustion process 9, steam process
It is divided into four systems: 10, fuel process 11, and ventilation process 12.

FIG. 3 is a configuration diagram showing the overall configuration of a thermal power plant. In the figure, reference numeral 41 is an air preheater for preheating combustion air with combustion exhaust gas, and 42 is a burner unit for adjusting the air-fuel ratio at each stage to perform denitration in the furnace.

302, 312, 322, 342 and 352 are wind box inlet air dampers for adjusting the combustion air amount of the burner stage. 30
1, 311, 321, 331, 341 and 351 are gas mixing dampers, which adjust the amount of combustion exhaust gas injected into the combustion air. Reference numerals 303, 313, 323 and 333 are primary gas dampers for adjusting the amount of combustion exhaust gas directly injected into the burner section. 43 is a condenser, 44 is a low-pressure feed water heater, 45 is a deaerator, 46
Is a high-pressure feed water heater, 47 is an evaporator, 48 is a primary superheater, 410
Is a tertiary superheater and 411 is a reheater.

FIG. 4 is a system diagram showing a combustion system of a denitration boiler in a furnace to which the present invention is applied.

The combustion system has four burner stages, and includes M1 burner 30, M2 burner 31, M3 burner 32, P burner 33,
It consists of two stages of after-airports 34 and 35. The M burner (main burner) is a main burner, and the P burner (planet burner) is an additional burner for reducing NOx.

Air dampers 302,3 on burners 30, 31, 32, 34 and 35 of each stage
12,322 and 332 and mixed gas dampers 301,311,321 and
331 and primary gas dampers 303, 313, 323 and 333 are installed.

The air dampers 302, 312, 322, 332, 342 and 352 are M1 to 3 burners 30, 31, 32 and P burner 33, after air port 34,
Adjust the air flow rate of 35. The air flow rate of the M1 to 3 burners 30, 31, 32 and the P burner 33 is adjusted by adjusting the air damper at the inlet of each stage burner in accordance with the fuel amount of each stage burner to a specified value. .

The air flow rate to the after-air ports 34 and 35 should be the total air amount minus the amount of air supplied to the M1 to 3 burners 30, 31, 32, and the P burner 33, and the air-fuel ratio in the total sum Adjust.

Further, the flow rates of the air supplied to the M burners 30 to 32 and the P burner 33 are corrected by the air ratio from the NOx target value to obtain the commands of the air dampers 302 to 322,332.

The NOx value can be reduced by making the combustion state worse. Therefore, to reduce the NOx value, the air damper 302
~ 322,332 are operated in the closing direction to decrease the oxygen concentration supplied to the burner (30-33) stage.

The exhaust gas mixing dampers 301, 311, 321, 331 inject an amount of exhaust gas commensurate with the flow rate of fuel supplied to the burner stage and control the oxygen concentration of each stage air flow rate to a prescribed value. The command to the exhaust gas mixture dampers 301, 311, 321, 331 is obtained by correcting the NOx target value for the exhaust gas mixture ratio corresponding to the fuel air amount.

In order to reduce NOx, the combustion state is deteriorated as described above, and the exhaust gas mixing damper is operated in order to increase the exhaust gas mixing amount and decrease the oxygen concentration in the wind box air.
301,311,321,331,341,351 move in the opening direction.

The primary gas dampers 303, 313, 323, 333 serve to prolong the burner flame by supplying the burner flame with an amount of the exhaust gas commensurate with the flow rate of the fuel supplied to the burner (30, 31, 32, 33) stage, like the exhaust gas mixing damper. To reduce the NOx value, it is effective to lower the combustion temperature.By increasing the primary gas flow rate as described above, the concentration of oxygen required for combustion is lowered, and the combustion is slowed down. It also lowers the temperature. Also in the primary gas dampers 303, 313, 323, 333, the primary gas damper command value is calculated by adding the NOx target value correction to the primary gas injection amount commensurate with the fuel flow rate.

FIG. 5 is a block diagram showing the configuration of the plant automatic control system. In the figure, reference numeral 52 is a master controller, 53 is a steam process system controller, 54 is a fuel process system controller, 55 is a combustion process system controller, and 56 is a ventilation process controller.

520 is a main turbine speed control controller, 530 is a water supply pump control controller, 531 is a spray control controller, 540 is an M burner fuel flow rate control controller, and 541 is P.
Burner fuel flow rate controller, 550 is a controller that performs air / gas flow rate control and burner control for each burner stage, 551 is a controller that controls air / gas flow rate, 560
Is a forced draft fan controller and 561 is a gas recirculation controller. These elements 520 to 561 are device controllers, and a control system is configured for each system device of the plant.

The master controller 52 gives the boiler input command 51 from the plant load command 50 from the central power supply station to the respective system controllers 53 to 56 which obtain the boiler input command 51.

The steam process controller 53 obtains a feedwater flow rate command 532 that matches the boiler input command, and supplies it to the feedwater pump controller 530. Further, a spray flow rate command 533 for requesting the setting of the steam temperature from the boiler input command is given to the spray control controller 531.

The fuel process system controller 54 obtains a fuel flow rate command from the boiler input command, issues a fuel command to the M burners 30 to 31 and the P burner 32, and gives a total fuel flow rate command to each burner.

The combustion process system controller 55 obtains the burner number control command 552 for each stage from the boiler input command, and M
And the air flow rate command 553 of each stage corresponding to the fuel flow rate of the P burners 30 to 32 and 33 is obtained. Based on the burner number control command 552 for each stage and the air flow rate command 553, the controller 550 that performs burner control and air / gas flow rate control of each stage should keep the combustion state in a state suitable for plant operation. Control. Further, a compensating air-gas flow rate command 554 is obtained in order to control the total air flow rate, and is given to the air / gas flow rate control controller 551.

The ventilation process controller 56 obtains and controls the input damper command of the gas recirculation fan 8 and the command to the forced draft fan 7 from the boiler input command.

In the plant control system configured as described above, the in-core NOx removal boiler intends to realize ultra-low NOx operation by optimally controlling the air-fuel ratio at each stage. The NOx control system is included in the controllers 550, 550a, 550b, 550c and the air / gas flow rate control controllers 551, 551a that perform the burner control of each stage and the air / gas flow rate control.

FIG. 1 is a diagram showing a controller for performing burner control and air / gas flow rate control, and control contents of the air / gas flow rate control controller.

The air / gas flow rate control per stage with a burner is the fourth
Burner inlet air dampers 302, 312, 322, 332 and mixed gas dampers 301, 31 such as M burners 30 to 33 and P burner 33 shown in the figure.
Controlled by 1,321,331 and primary gas dampers 303,313,323,333.

The burner inlet air damper opening is calculated from the burner stage fuel flow rate 101 by the function generator 104 to find the theoretical air flow rate that matches the fuel flow rate being supplied to the relevant ghana stage, and the ECO outlet (Economizer: The charcoal is a charcoal heater that heats the feed water with exhaust gas, and the ECO output refers to the boiler outlet.) The multiplier 105 performs correction to keep the oxygen concentration of the gas at a specified value, and then , Boiler input command 103
From the NOx target index 102 and NOx target index 102, the air ratio correction signal 106 that is most suitable for the current combustion state is obtained from the air ratio calculator 100, and this correction is performed by the multiplier 107 to obtain the burner inlet air flow rate command value. The difference between this command value and the air flow rate 108 of the burner stage is calculated by the subtractor 109, and the proportional-plus-integral calculator 110 performs proportional-plus-integral calculation on the difference.

When reducing NOx, the burner inlet air dampers 302, 312, 322, 332 operate in the closing direction. Therefore, when the NOx target index is lowered, the air ratio correction signal is reduced and the burner inlet air flow rate command value is operated in the lowering direction.

The mixed gas dampers 301, 311, 321, 331 mix the exhaust gas with the burner inlet air in order to keep the oxygen concentration in the burner air box at a specified value.The air flow rate is adjusted so that the exhaust gas oxygen concentration is a value determined by each load. Inlet air damper 302
Since it is controlled by the after-air dampers 342 and 352, similar to the burner inlet air damper, the mixed gas flow rate target value is obtained by the function generator 120 from the theoretical air amount supplied to the burner stage, and further the NOx target index and the boiler are set. GM ratio (gas mixing ratio: ratio of mixed gas) correction signal from input command
The correction of 121 is performed by the multiplier 122, and the opening degree of the mixed gas damper is determined by proportional integral calculation from the deviation between the mixed gas flow rate command value and the mixed gas flow rate.

When reducing the mixed gas damper and NOx, it operates in the open direction and operates in the direction to reduce the oxygen concentration in the air box. Therefore, NOx
When the target index is lowered, the mixed gas flow rate command value is increased.

The primary gas dampers 303, 313, 323, 333 obtain the primary gas flow rate set value with the function generator 130 based on the fuel supplied to the stage burner which is the burner load, and the PG ratio (primary gas ratio: primary gas ratio: primary gas ratio: The ratio of primary gas) The correction signal 131 is corrected by the multiplier 132 to obtain the primary gas flow rate command value.

The opening degree of the primary gas damper is determined by the proportional-plus-integral calculation from the deviation between this command value and the primary gas flow rate.

The primary gas dampers 303, 313, 323, 333 provide a primary gas that forms a layer between the fuel and a mixed gas of air and exhaust gas, and when reducing NOx, increase the amount of this primary gas to slow down the combustion reaction and increase the combustion temperature. It works in the lowering direction, and when the NOx target index is lowered, it operates in the direction of increasing the primary gas flow rate, so the primary gas damper operates in the opening direction.

After-air port dampers 342 and 352 reduce the after-air port air flow rate target value with the remainder after subtracting the sum of the burner inlet air flow rate commands for the M and P burner stages from the total air flow rate command, and set the opening to match it. To do.

That is, the total air flow rate target value 140 is corrected by the multiplier 141 to compensate the oxygen concentration of the ECO outlet gas to the specified value, and the total air flow rate command value 143 is calculated. And the sum of the burner inlet air flow rate command values of the P burner 33 is subtracted by the subtractor 142 to calculate the after air port air flow rate target value.

For the after air port 342 in the lower stage with respect to this after air port air flow rate target value, the multiplier 144 corrects the AA ratio correction signal 143 from the NOx target index and the boiler input command, and the after air port air flow rate command value is calculated. Ask. By performing proportional integral calculation based on the deviation between this command value and the after-air port air flow rate, the after-air port damper 34
Determine the opening of 2.

On the other hand, for the after-air port air flow rate command value of the upper stage, the lower-stage after-air port air command value is obtained by subtracting the upper-stage after-air port air command value with AA ratio correction from the after-air port air target value by the subtractor 145. The opening degree of the after-air port damper 352 is determined by proportionally integrating the deviation between the command value and the lower after-air port air flow rate.

Here, the NOx target index 102 and the function of obtaining each air / gas ratio correction from the boiler input command 103 are shown in FIGS. 6 and 7.
It will be described with reference to the drawings.

The function generators 60, 61, 62 indicate the air ratio correction amounts for the NOx master (NOx target index 102) 0%, 50%, 100% with respect to the boiler input command. NO for air ratio and AA ratio correction
x Set the master 100% to a large value and 0% to a small value. The subtracters 65 and 66 are air ratios when the NOx master is 0 to 50%,
AA ratio correction amount deviation, air ratio at NOx master 50-100%, AA
Indicates the deviation of the ratio correction amount.

Depending on the value of the NOx master, these deviations are given gains by the multipliers 63 and 64 as shown in FIGS. 8 and 9, and the deviation is interpolated from the function generator 61 when the NOx master is 50%. Give the deviation.

The deviation from the NOx master 50% time obtained by the above interpolation method is added to the output of the function generator 61 by the adder 67 to obtain the air ratio,
Obtain the AA ratio correction signal.

On the other hand, in the GM ratio and PG ratio correction signals, since the characteristics of the GM ratio and PG ratio correction signals with respect to the NOx master signal are opposite, the settings of the function generators 60 and 62 are reversed (10th).
Figure). This can be seen from the relationship between the air amount and the NOx value of the exhaust gas amount.

The setting of the NOx master can be continuously set between 0 and 100%, and 0% indicates that the NOx value is low and 100% indicates that the NOx value is high.

〔The invention's effect〕

As described above, according to the present invention, the NOx target value can be precisely controlled, and NOx can be reduced while operating in a stable combustion state.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a configuration diagram showing a schematic configuration of a thermal power plant, FIG. 3 is a configuration diagram showing the overall configuration of the plant, and FIG. 4 is a boiler air. A system diagram showing a gas flue airway, FIG. 5 is a configuration diagram showing an automatic plant control device, FIG. 6 is a circuit diagram showing an air / gas correction circuit, and FIG. 7 is a diagram showing load-air ratio characteristics, 8 and 9 are graphs showing NOx master value characteristics in air ratio,
FIG. 10 is a diagram showing load-air ratio characteristics. 1 ... Boiler, 2 ... Turbine, 3 ... Generator, 30-31
…… M burner, 32 …… P burner.

Claims (1)

[Claims] 1. A method for reducing nitrogen oxides by introducing combustion air and exhaust gas into a combustion chamber and changing the composition of these to change the combustion state of the burner section, which is supplied to each stage burner. In the furnace denitration control method for variably controlling each flow rate of the mixed gas taken out from the combustion air and the exhaust gas, and the primary gas, the burner control and the air and gas flow rate control are integrated for each stage burner. And stored in the controller, the air ratio to the load, the air ratio to the after air port, by setting some target index of nitrogen oxides against this, and interpolating between them, The flow rate values of the combustion air, the mixed gas, and the primary gas for the load and the nitrogen oxide target index are set, and the flow rates are set so as to reach the set flow rate values. A method for controlling denitration in a furnace, which comprises controlling